In the developmental process of WO3 based transmissive ECDs, we are concentrating our efforts for developing ion storage counter electrode films with a high transmittance for visible light both in the charged and discharged state and an ion storage capacity exceeding 20 mCcm−2 or comparable to WO3 to provide sufficient number of ions for deep colouration. Therefore, work has been undertaken in this direction for the synthesis of CeO2 precursors doped with TiO2 via a wet chemistry route.
The widely used sol-gel process offers numerous advantages over the other conventional deposition techniques, which include tailor making of the film's properties, introduction of porosity in the films, low process cost and possible processing at low temperature. Preparation of CeO2 based films by sol-gel technique has been attempted following various routes. The use of alkoxides, the most popular precursor material in sol-gel processing has been reported by D. Keomany, C. Poinsignon, D. Deroo. in Sol. Energy Mater. Sol. Cells 33 (1994) 429-441. Alternately salts of cerium, like CeCl3.7H2O, [(NH4)2{Ce(NO3)6}] in combination with Ti alkoxides have been shown by A. Makishima, M. Asami and K. Wada, in J. Non-Cryst Solids 121 (1990) 310-314 as one of the routes to get CeO2—TiO2 films. Based on the earlier reports on these materials, A. Makishima, M. Asami and K. Wada, in J. Non-Cryst. Solids 121 (1990) 310-314 have performed a study in which the type of alkoxyl group of titanium alkoxide and the kind of catalyst have been varied in order to study their influence on the properties of the films. The films deposited by the authors have been sintered at 500° C. and the XRD patterns of these films are characterized by the appearance of diffraction peaks of the CeO2 phase alone. In our earlier invention, we have reported the preparation of CeTi2O6 compound in thin film form using a sol-gel process employing the same precursor materials as in the present invention. CeTi2O6 thin films have the potential to be used as passive counter electrodes in electrochromic devices.
In the sol-gel process, using alkoxide compounds, hydrolysis and condensation reactions are crucial for obtaining a gel. Through hydroxylation-condensation reactions, oxopolymers from transition metal alkoxides (TMA) can be grown into an oxide network as has been reported by D. C. Bradley, R. C. Mehrotra and D. P. Gaur in Metal Alkoxides (Academic Press, New York, 1978). The normal course of the reaction for transition metal alkoxides dissolved in a solvent leads to precipitation of the polymers. Control of the reactivity of TMA is necessary in order to obtain sols and gels. In titanium-based systems, this control is achieved through the addition of complexing agents and salts such as ceric ammonium nitrate and cerium chloride.
Photocatalytic reaction sensitized by TiO2 and other semiconducting materials has attracted extensive interest as a potential way of solving energy and environmental issues. Several cerium titanates have been investigated for photocatalytic activity. Yellow colored cerium titanate, CeTi2O6 with mainly Ce4+ state is known to cause photobleaching of methylene blue aqueous solution with irradiation of Xe discharge light as reported by S. O-Y-Matsuo, T. Omata, M. Yoshimura in J. Alloys and Compounds, 376 (2004) 262-267. Mixed CeO2—TiO2 films are reported by Q. N. Zhao, C. L. Li, X. J. Zhao in Key Engineering Materials 249 (2003) 451-456 to decolorize methyl orange solutions upon irradiation of the UV light.
Brannerite, UTi2O6 is an accessory phase in the titanate-based crystalline ceramics of synroc as reported by A. E. Ringwood, S. E. Kession, N. G. Ware, W. Hibberson, A. Major in Nature (London) 278 (1979) 219. The ideal formula of natural brannerite is (U,Th)1−xTi2+xO6 with a uranium deficiency and excess titanium. Possible cation substitutions identified in natural brannerite for uranium are Pb, Ca, Th, Y, Ce and for titanium are Si, Al, Fe. Stoichiometric brannerite is monoclinic with space group C2/m. There are two different distorted octahedra in CeTi2O6 structure. Distorted TiO6 octahedra form a zigzag sheet by sharing common edges, and each Ti octahedron shares three edges with titanium octahedra and three corners with cerium octahedra. The sheets of TiO6 octahedra are identical with those of the anatase structure parallel to (101) plane. The Ce cations located at the interlayer sites connect adjacent sheets. Each cerium octahedron shares two common edges with neighboring cerium octahedra and four corners with TiO6 octahedra. As has been reported by K. B. Helean, A. Navrotsky, G. R. Lumpkin, M. Colella, J. Lian, R. C. Ewing, B. Ebbinghaus and J. G. Catalano in J. Nucl. Mater. 320 (2003) 231-244, CeTi2O6 in the powdered form can be prepared by sintering in air (at 1350° C. for >100 h) a pellet containing stoichiometric portions of the oxides, CeO2 and TiO2. The preparation of CeTi2O6 solid solution using cerous nitrate and titanium tetrachloride precursors has been reported by Y. Chen, X. Jiang and L. Lou in CN1565724. As reported by Chen Linchen, Lu Guanglie, and Hu Xiurong in J. Rare Earths, 21 (2003) 108-111, the formation of CeTi2O6 powder using the sol-gel process is possible using precursor materials, Ce(NO3)3.6H2O, Ti(OBu)4 in anhydrous ethanol. The present invention reports the synthesis of CeTi2O6 compound from different precursors i.e. cerium chloride heptahydrate and titanium propoxide.
In the present invention, the CeTi2O6 phase has been achieved in powdered form by the sol-gel technique, which represents a reliable, low-cost chemical route. In comparison to the powdered CeTi2O6 material, which is formed by other research groups by ball milling stoichiometric portions of CeO2 and TiO2, the same material in the present invention is prepared by a simple sol-gel process wherein homogeneous solutions containing different precursors can be prepared at relative ease and greater precision. CeTi2O6 compound has the potential for use as a photocatalytic agent. The CeTi2O6 compound in the present invention has shown superior response as a photocatalyst in comparison to TiO2 powder, which is the best known photocatalytic agent as per the literature reports. Using cerium chloride heptahydrate and titanium propoxide precursors, we have reported earlier in Sol. Ener. Mater. Sol. Cells 86 (2005) 85-103, the formation of a mixed compound of CeO2 and TiO2 i.e. CeO1.6.2TiO2 in thin films at annealing temperature of 500° C. from the Ce:Ti compositions, 4:1 and 2:1.